The world-wide production of 2-ethylhexanol-1, which is prepared from 2-ethyl-2-hexenal, is greater than all alcohols other than those containing from 1 to 4 carbon atoms, due mainly to the widespread use of its carboxylic acid esters as a plasticizer, especially in polyvinylchloride. Other uses of this 8-carbon alcohol include the production of intermediates for acrylic surface coatings, diesel fuel and lube oil additives, and surfactants. 2-Ethylhexanol-1 is prepared from n-butyraldehyde as the feedstock, where the latter is the highest volume oxide chemical produced, via the aldol condensation of n-butyraldehyde to 2-ethyl-2-hexenal followed by reduction of both the olefin and aldehyde moieties, ##STR1## where the actual aldol condensation is represented by the conversion I to II.
The aldol condensation of aldehydes is a well known and time honored reaction employed for many years in the production of several commercially important materials in addition to 2-ethylhexanol-1, for example, the formation of isophorone and mesitylene oxide from acetone. The reaction is not merely base catalyzed, but usually needs a strong base catalyst in order to proceed satisfactorily. Although the aldol product corresponding to II may often be isolated, its dehydration to III is usually facile under the reaction conditions, and accordingly it is most frequently the alpha, beta-unsaturated aldehyde III which is the isolated reaction product.
Often the strong bases used as catalysts in aldol condensation are the alkali metal hydroxides, especially under aqueous or partly aqueous conditions. It should be apparent that the use of alkali metal hydroxides does not lend itself to the adaptation of aldol condensation as a continuous process, in large part because of the hydroxides having unfavorable properties when used as a fixed bed. Yet development of a continuous process for the production of 2-ethyl-2-hexenal and other aldol condensation products is not merely of great interest but rather is of high priority, because of the well-known advantages of fixed bed continuous processes generally and because it would minimize environmental problems associated with the disposal of a strong base as well as minimizing corrosion difficulties caused by a strong aqueous base.
The desirability of a strong base suitable for use as a fixed bed previously has been recognized and has led to the use of such materials, inter alia, as sodium on alumina and potassium on graphite. Because of the severe limitations of such strong bases in a fixed bed, more recent attention has turned to clays and clay-like materials as suitable alternatives.
Hydrotalcite is a clay with the ideal unit cell formula of Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O, and closely related analogs with variable magnesium/aluminum ratios may be readily prepared. Nakatsuka et al., Bull. Chem. Soc. Japan, 52, 2449 (1979) has described the catalytic use of "calcined synthetic hydrotalcite" with varying molar ratios of MgO/Al.sub.2 O.sub.3 in the batch mode polymerization of beta-propylactone. More extensive work was reported later on the use of "synthetic hydrotalcite" in various base-catalyzed reactions by W. T. Reichle, J. of Catalysis, 94, 547 (1985), who found that aldol condensations in a pulse reactor were readily catalyzed by "synthetic hydrotalcite" compositions having Mg/Al ratios from 1.3 to 6.3, although the Mg/Al ratio did not appear to have a significant effect on either its catalytic activity or efficiency. From deuterium exchange studies Reichle also concluded that the pK.sub.a of hydrotalcite was between 35 and 45. E. Suzuki and Y. Ono, Bull. Chem. Soc, Japan, 61, 1008 (1988 ), reported on the aldol condensation between formaldehyde and acetone using as catalysts two quite different types of hydrotalcite-like materials, both being derived from hydrotalcite itself. In one series of catalysts the carbonate moiety of hydrotalcite was exchanged by NO.sub.3.sup.-, SO.sub.4.sup.2-, or CrO.sub.4.sup.2-, and in the other series there was isomorphous substitution of Mg.sup.2+ --Al.sup.3+ by Li.sup.+ --Al.sup.3+, Co.sup.2+ --Al.sup.3+, Ni.sup.2+ --Al.sup.3+, or Zn.sup.2+ --Cr.sup.3+. At 500.degree. C. reaction temperature none of the foregoing appeared to lead to increased acetone conversion although some slight increase in selectivity (especially at lower conversion) was observed. Nunan et al., J. of Catalysis, 116, 222 (1989), has prepared related materials by isomorphous substitution of Mg by Cu and Zn, and of Al by Cr or Ga.
Before proceeding it appears advisable to prevent semantic obfuscation by defining several terms, using first a specific example and then generalizing by analogy. Although "hydrotalcite" is most properly applied to a clay of composition Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O often it has been used to describe related layered double hydroxides with varying Mg/Al ratios. However, after calcination of the layered double hydroxides the resulting materials are better described as solid solutions of magnesium oxide and aluminum oxide. That is, calcination destroys the layered structure characteristic of hydrotalcite and affords a solid solution. But the terminology as applied to such solid solutions often retains the "hydrotalcite" name, as in, for example, "synthetic hydrotalcites". In this application henceforth we shall try to be consistent in using the term "solid solution" of, e.g., magnesium oxide and aluminum oxide, to describe such calcined synthetic materials. The second point involves the use of the term "Mg/Al". In this application Mg/Al shall be the number ratio of magnesium to aluminum atoms in a solid solution of magnesium oxide and aluminum oxide. While this definition has been previously employed by, for example, Reichle, others have used a different definition for the Mg/Al ratio.
We can generalize the foregoing characterization to the family of materials, described more fully within, having the general formula (M.sup.a).sub.x (M.sup.b).sub.y (OH).sub.z A.sub.q.rH.sub.2 O, where M.sup.a is a divalent metal or combination of divalent metals, M.sup.b is a trivalent metal or a combination of trivalent metals, and A is an anion, often carbonate. Such materials are layered double hydroxides. However, after calcination of the layered double hydroxide the resulting product is a solid solution of the two oxides, M.sup.a O and M.sup.b.sub.2 O.sub.3. We shall retain this distinction between layered double hydroxides and solid solutions throughout this application.
Shortly after Reichle's work, Corma and coworkers described their investigations into the use of certain zeolites as base catalysts; A. Corma and coworkers, Applied Catalysis, 59, 237 (1990). The effect of a series of alkali metal exchanged X and Y zeolites was investigated in batch reactions in catalyzing the condensation of benzaldehyde with ethylcyanoacetate and diethyl malonate, where it was determined that the reactivity of the metal-exchanged zeolites was in the order cesium&gt;potassium&gt;sodium&gt;lithium, and X&gt;Y. The pK.sub.b 's of these materials were said to be between about 10.3 and 13, which is far less than that given by Reichle for his "synthetic hydrotalcites". In related work [A. Corma and R. N. Martin-Aranda, J. Catalysis, 130, 130 (1991)], Corma exchanged the magnesium ions on the edges of the octahedral sheet in sepiolite with alkali metal ions to afford materials also effective as base catalysts in the foregoing condensation, but noted that the basicity of the resulting materials also was far less than that of hydrotalcite.
Our objective was the development of a continuous process for the aldol condensation of suitable carbonyl compounds, especially in the liquid phase. It was important that the process be continuous and employ a fixed bed of catalyst. Therefore the catalyst had to possess suitable flow properties, compressibility, and so forth, consistent with a liquid flow. It is important to note that whereas some aldol condensations, such as the condensation of n-butyraldehyde, are performed in the vapor phase, it was quite desirable that the new process also be applicable to liquid phase aldol condensation. It also is important that the aldol condensation proceed in relatively high yield, with good selectivity, and at modest temperatures, say less than 200.degree. C. Since water is a reaction product, it is important that the catalysts exhibit hydrothermal stability. We have found that solid solutions of a divalent metal oxide and a trivalent metal oxide with high surface area appear to satisfy the foregoing criteria in all respects.